This invention has to do with orthopedic prosthetics. More specifically, this invention relates to prosthetic joints, and more particularly, to prostheses for replacement of dysfunctional knee joints.
Orthopedic prostheses of the type with respect to which the present invention is particularly useful are disclosed in our U.S. Pat. No. 4,309,778 for NEW JERSEY MENISCAL BEARING KNEE REPLACEMENT and U.S. Pat. No. 4,470,158 for JOINT ENDO PROSTHESIS. The subject matter of these patents is particularly relevant and is incorporated herein by reference.
Prostheses structured in accordance with the teachings of our prior patents have been singularly successful, particularly when used as knee replacements for dysfunctional knees. From time-to-time, however, the prostheses have been implanted with less attention to proper ligamentous tension than is necessary for proper operation. Under such circumstances, when certain loading conditions occur on the prosthesis, such as the loads experienced when the patient arises from a chair, subluxation can occur.
Considering initially a rotating bearing knee structure, according to U.S. Pat. No. 4,470,158, non-uniform loading often occurs when the patient moves from seated to a standing position. Typically, the stresses which are generated during movement from seated to a standing position tend to displace the distal femur anteriorly with respect to the proximal tibia. With the rotating bearing knee prosthesis in place, there are no cruciate ligaments and therefore there is no anterior-posterior stability independent of the prosthesis. If the prosthesis was implanted without proper attention to ligamentous tension, the introduction of medial lateral forces tending to cause varus-valgus torque may tend to lift one of the femoral condylar surfaces off the rotating bearing element. If the lift is sufficient to permit movement of the rotating bearing under the femoral condylar surface, then subluxation may occur.
With respect to meniscal bearing knee replacement of the type shown in U.S. Pat. No. 4,309,778, the problem is not so often observed because the medial and lateral bearing elements are not connected and a certain amount of cruciate ligamenture is maintained. However, shear stresses such as those attendant to moving from seated to standing position when coupled with varus-valgus torque which lifts one of the femoral condylar surfaces off the bearing element may also result in subluxation.
Obviously, with respect to either prosthesis construction, pure shear stresses of sufficient magnitude may be sufficient to cause subluxation. Ordinarily, such forces are directed anteriorly as a result of stresses occurring during natural but extreme flexion.
The occurrence of subluxation as a result of shear stresses is a problem which has been recognized in the field. One approach to reducing the occurrence of subluxation has been to provide posterior stabilization. Prostheses constructed pursuant to this approach utilize a spine formed on the tibially mounted bearing, which is intended to cooperate with a cam surface formed in the femoral element to reduce the likelihood of subluxation. It has been found, however, that the requirement to form a cavity in the femur to accomodate the design creates problesm, and flexion is limited such that the function of the prosthesis does not correspond to that of the natural joint. Further, the tibial spine element of such designs acts as a lever arm which tends to lift the tibial bearing off its platform by rotation. Even the stresses from normal activities create loads which may be unacceptable, sometimes contributing to loosening of the tibial element from the tibia itself.
It is an object of the present invention, therefore, to provide a bearing for an orthopedic prosthesis which reduces the likelihood of subluxation.
A further object of the present invention is to provide a joint prosthesis with at least one bearing element structured to reduce the likelihood of subluxation.
Yet another object of the present invention is to provide a bearing for a joint prosthesis which is useful with presently known joint prostheses.